Vertebrates are characterized by possession of hypobranchial muscles (HBMs). Cyclostomes, or modern jawless vertebrates, possess a rudimentary and superficial HBM lateral to the pharynx, whereas the HBM in jawed vertebrates is internalized and anteroposteriorly specified. Precursor cells of the HBM, marked by expression of Lbx1, originate from somites and undergo extensive migration before becoming innervated by the hypoglossal nerve. How the complex form of HBM arose in evolution is relevant to the establishment of the vertebrate body plan, but despite having long been assumed to be similar to that of limb muscles, modification of developmental mechanisms of HBM remains enigmatic. Here we characterize the expression of Lbx genes in lamprey and hagfish (cyclostomes) and catshark (gnathostome; jawed vertebrates). We show that the expression patterns of the single cyclostome Lbx homologue, Lbx-A, do not resemble the somitic expression of mammalian Lbx1. Disruption of Lbx-A revealed that LjLbx-A is required for the formation of both HBM and body wall muscles, likely due to the insufficient extension of precursor cells rather than to hindered muscle differentiation. Both homologues of Lbx in the catshark were expressed in the somitic muscle primordia, unlike in amniotes. During catshark embryogenesis, Lbx2 is expressed in the caudal HBM as well as in the abdominal rectus muscle, similar to lamprey Lbx-A, whereas Lbx1 marks the rostral HBM and pectoral fin muscle. We conclude that the vertebrate HBM primarily emerged as a specialized somatic muscle to cover the pharynx, and the anterior internalized HBM of the gnathostomes is likely a novelty added rostral to the cyclostome-like HBM, for which duplication and functionalization of Lbx genes would have been a prerequisite.

中文翻译：

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脊椎动物下鳃肌进化的新发育基础。


脊椎动物的特点是拥有下臂肌（HBM）。圆口动物，或现代无颌脊椎动物，在咽部外侧具有基本的浅表 HBM，而有颌脊椎动物的 HBM 是内化的且前后特定的。 HBM 的前体细胞以 Lbx1 的表达为标志，起源于体节，并在受到舌下神经支配之前经历广泛的迁移。 HBM 的复杂形式如何在进化中出现与脊椎动物身体计划的建立有关，但尽管长期以来人们认为 HBM 与四肢肌肉相似，但 HBM 发育机制的改变仍然是个谜。在这里，我们描述了七鳃鳗、盲鳗（圆口动物）和猫鲨（颌口动物；有颌脊椎动物）中 Lbx 基因的表达。我们发现，单个环口虫 Lbx 同源物 Lbx-A 的表达模式与哺乳动物 Lbx1 的体细胞表达不同。 Lbx-A 的破坏表明 LjLbx-A 是 HBM 和体壁肌肉形成所必需的，这可能是由于前体细胞延伸不足而不是肌肉分化受阻。与羊膜动物不同，猫鲨中 Lbx 的两种同源物均在体节肌原基中表达。在猫鲨胚胎发生过程中，Lbx2 在尾部 HBM 以及腹直肌中表达，与七鳃鳗 Lbx-A 类似，而 Lbx1 则标记头侧 HBM 和胸鳍肌肉。我们得出的结论是，脊椎动物的 HBM 主要是作为覆盖咽部的特殊体细胞肌肉出现的，颌口动物的前部内化 HBM 可能是环口类 HBM 的新奇附加喙部，为此 Lbx 基因的复制和功能化可能是一个先决条件。